The exposure of metallic, concrete and plastic surfaces to chloride ion results in contamination with chloride ion, chloride derivatives and flash rust (metal oxide), and in corrosion of associated surfaces. Chloride contamination eventually leads to destructive results such as coating adhesion failure, cement cracking due to rebar corrosion, and the like.
Water soluble chloride salt contamination of surfaces, particularly steel and iron surfaces, has always been a major cause of coating failure. Coating failure or debonding is initiated by substrate corrosion due to residual chloride presence which results in ferrous chloride being formed under the coating surface. If oxygen is present, the ferrous chloride is oxidized to ferric chloride which absorbs moisture from the air and forms a concentrated ferric chloride solution. This solution causes electrochemical corrosion and rusting under the coating and ultimately results in coating failure. In general, chloride contaminants must be completely removed from the surface to avoid future coating failures.
Chloride contamination of rebar in concrete poses another serious problem. If the rebar becomes contaminated with chloride, corrosion products will build up on the rebar within the concrete. These corrosion products may have up to four times the volume of the original rebar, resulting in stressing and eventually cracking and deterioration of the concrete, rendering it substandard or unsatisfactory for its intended use.
Furthermore, flash rust (metal oxide) may develop on metal surfaces contaminated with chloride, further promoting coating failure and adding undesirable color.
No standards exist as to the particular level of chloride ion or soluble salts that can be tolerated by particular surfaces in various applications, and performance of the surfaces depends quite heavily on the particular environment. For example, some of the most difficult applications include off-shore oil drilling rig platforms, and highways heavily salted to remove ice and snow during the winter months. Furthermore, some coatings are porous, providing increased opportunity for chloride contamination and subsequent coating failure. Given the variety of applications in which metallic, concrete and plastic surfaces are used, and the lack of a standard as to the tolerable level of chloride ion contamination, the best performance for any coating or environment will result if the surface is free of chloride ion or soluble salts prior to the application of the coating.
Various methods have been employed to remove chloride ion and other scale-like derivatives from contaminated surfaces, with each method having its own drawbacks. For example, high or ultra high pressure washing (i.e., up to 20,000 psig) and water blasting with hard grit both require expensive equipment as well as workers who are fully trained in the proper use of the equipment. Also, when the contaminated surface is loaded with heavy scale, this scale can prevent the water jet from dissolving the underlying chloride salts, and these methods typically do not result in the complete removal of chloride ion or chloride derivatives.
Mineral acid washing is also employed, followed by water rinsing. However, this process is not always effective and may leave residues on the surface unless the surface is washed extensively with deionized or distilled water. Also, the process may result in flash rusting. Furthermore, acids are particularly dangerous when used in enclosed areas such as tanks, etc., and also impose an environmental hazard and disposal problems.
Dry blasting with an air driven abrasive followed by water rinsing also has been utilized. However, several treatments usually are required to achieve a substantial reduction in residual chloride contamination on the treated surface which has been cleaned of other scale-like deposits such as iron oxide or rust. The process is labor intensive and time consuming and requires a considerable amount of abrasive, all of which add to the cost. Also, sand blasting may be used to remove old coatings and rust, but usually it does not remove any substantial amount of the chloride contamination.
Recently, an aqueous jet blasting method employing a sodium bicarbonate soft grit abrasive followed by a high purity water rinse has been claimed to achieve "zero detectable" salt concentrations on the substrate being cleaned (National Corrosion Engineers, T-14 Unit Committee Meeting, Mar. 9, 1993, "A New Method for Achieving Zero Detectable Soluble Salt Contamination on Substrates", by Loren L. Hatle and J. R. Cook). This process still requires proper ventilation for the workers and the disposal of the abrasive, along with precise control of the bicarbonate abrasive composition and use of high purity water in order to achieve low residual chloride levels.
Therefore, it is desirable to have a method for removing chloride ion or other corrosive scale-like derivatives from a contaminated surface where the method is effective yet simple to use.